The present invention generally relates to inlet doors and, more particularly, to pre-hung air inlet door systems for gas turbine engines, such as auxiliary power units (APU).
Many modern aircraft are equipped with an airborne APU that provides electrical and pneumatic power to various parts of the aircraft for such tasks as environmental control, lighting, powering electronics, main engine starting, etc. FIG. 1 shows an APU 20 installed in the tailcone of an aircraft 25. In order to utilize the APU 20, various installation systems must be included with the APU 20. Some of the principal systems may include a mount system 21, an inlet system 22, an exhaust system 23, and a pneumatic system 24.
The inlet system 22 for the APU 20 conveys air from outside the aircraft 25 to the APU compressor to be used for combustion and extraction of work. A key component of the inlet system 22 is the air inlet door 26. The air inlet door 26 provides ram recovery for in-flight APU starting and operation, a low loss (pressure) opening for APU operation on the ground, and Foreign Object Damage (“FOD”) protection while the APU 20 is not operating and the air inlet door 26 is in the closed position.
U.S. Pat. No. 6,349,899 provides an air inlet mechanism for an aircraft. The mechanism includes, among other components, a housing that forms a fluid conduit. The housing is coupled to an aircraft structure at one end and coupled to a duct structure at the other end. The housing supports a hinge pin that allows an air inlet door to rotate relative to the housing. Motion is imparted on the air inlet door via a rotary actuator coupled to the housing and the hinge pin. The hinge pin extends from the actuator on one wall of the housing, through both side walls of the air inlet door, and through the opposite wall of the housing. Although the described air inlet mechanism can convey air from outside the aircraft to the APU compressor, installation and repair of the air inlet mechanism is time consuming and expensive. Indeed for some small aircraft the engine must be removed to allow access to the components of the air inlet mechanism when repairs are necessary.
The close tolerance interface required between the aircraft skin, the fixed structure of the aircraft, and the motive air inlet door has been difficult to achieve with current designs and current assembly practices. Shimming, hand trimming, and special rigging procedures result in excessive assembly time and often prevent component interchangeability. Binding and misalignment of rotating components are frequently responsible for premature failures of costly air inlet door components such as actuation systems and bearings.
As can be seen, there is a need for an air inlet mechanism wherein assembly time is reduced and component interchangeability is increased. Additionally, an air inlet assembly is needed that can be accessed for repair without removing the engine.